Secret signaling



Patented June 16, 1 925.

uNiT-o STES AXE-N.

ROBERT c. MATI-IES, OF new YORK, N. Y., AssIenoR o WESTERN ELEcTRIccol v1- PANY, INCORPORATED, OF N W YORK, Y., A CORPORATION or NEwvoRx.

SECRET SIGNALING.

Application filed June 30,1923. Serial No. 648,648.

To all whom it may concern:

- Be it known that I, ROBERT C. MATHES, a citizen of the United States of America, residing at New York city, in the county of Bronx and State of New York, have invented certain new and useful Improvements in Secret Signaling, of which the following is a full, clear, concise,'and exact description.

The present invention relates to the secret transmission of waves for any desired pur pose, such as for signaling.

The invention is particularly applicable to the transmission and reception of waves,

the components of which extend over. acon-- 3 'siderable frequencyv range and have .a char- =acteristic frequency order. as, -for example, speech waves. The invention takes advantage of the fact that any: material alteration in the order ofoccurren'ce of the various frequency components of the wave makes it difliciilt to restore the wave to its-character- :istic form so that an unauthoriz edpersoni who may intercept the wave being secretly transmitted is unable'to restore it to 'recog-- nizable for understandableform.

An object of the, invention 18 to secure a.- jhigh degree of secrecyinwave transmisi sion with com aratively simple apparatus.

: .A further. 0 ject'is to enable secret radio communication to be-efiecte'd with the ex-' penditure of no more energyv than is necessary for non-secret communicatlom Briefly and spec'fically described, the invention 'comprisesisubdividing the waves to be "transmitted 'asto their frequency components, into sub-bands of frequencies, and

producing a%phase 'or time shift in ,some of -..the subebands with respect to the others before transmission. In-the specific embodiment of the invention to be described, this time shift isproduced by the .use of' .3.

storage device. The various sub-bands are then, recombined andtransmitted at their own frequencies or they may first be shifted 4 to adifferent frequency level and then be transmitted. Each-of the sub-bands'is preferablylof lsuch narrow frequency range as to be incapable 'gof' reproducing the speech or otherwave being sent, so that it is im-. possible to receive the signal by filtering out'any one, 'ofthe sub-Hands from the others. At the receiving station employing the invention, the" various sub-bands 'are separated filtering, and such-a time shift' Fig. "1' shows schematically a transmitting station employing the invention; 2 shows a receiving station according to the invention for cooperating with the station of- Fig. 1 and Fig. 3 shows a'form of storage device suitable for use in making-the time shifts in sub-bands.

the transmitted and received- In the-system of Fig. 1, speech currents, I

present in the circuit 2- as a result of talking into the transmitterl, are impressed I upon the three .analyzingfilters- F F and to transmit adjacent portions of the speech :frequency' band. In'fthe arrangement shown, .forexample, the 'filt'e'r F, may transmit fr'equencies from 100 to 800 cycles per second.

; Filt e'r F may select the frequenciesSOO to 1500 cycles; and filter F, the frequ encies 115.00 to '2200, .on the assumption that-the frequencies 100 to 2200-are to be transmit- Thesefilters are preferably band'filter's havingtheir constants so proportioned as' ted A'.difi'erent number of filte rs' than the three shown may be used, if desired, and the frequency range -"ofeach filter will deance with the principles disclosed in the patent'to Campbell 1,227,113, May-22, 1917. The frequency components selectively pressed upon the respective timelag devices TL TL,',' -'and- TL These'devices are ar-. ranged in a manner; to be described pres- ;ently. -so "as to introduce a .difi'erent 'tiine lag in "the transmission :offthesever'al subbands selectedlby the analyzing filters. Thesub-bands with their differentti'me lags are then impressed 'in com'mon upon 'the circuit .3 from which-they are applied'to the transmitting. modulatof M. A source 4 of 105 lator M, s'ot'hat the impressed sub-bandsmodulate the, carrier wave andthe moduc'arrier waves is also coupled-with the1modu1 .pend upon the number of filters'used. -Each "of these filtersmay'lbe designed inaccord-v transmitted by the, several filters are im- V '5,- 9

lated waves are impressed uponithe tr'ansmitting antenna- TA. The modulator M is lp re aferablyev of the balanced tube type 'shown sub-bands.

a are applied to the circuit 3 of Fig. 1.

the outgoing circuit 3.

inthe patent to Carson 1,343,306, June 15,-

1920, and the carrier source may be an oscillatorof the type shown in the Hartley Patent 1,356,763, October 26, 1920. This type'of modulator has the property of suppressing transmission of the unmodulated carrier wave component so that only the two side bands resulting from the modulation of the carrier by the applied sub-bands are present in the output of the modulator M. The breadth ofthe frequency band transmittedmay be reduced by tuning the antenna TA toone of the two side bands or in the case of relatively high radio frequencies where' it would be impracticable to suppress one of the speech bands by tuning the antenna, the modulation system disclosed in the pat ent to Osborne 1,361,488, December 7, 1920, may be employed to 'secure single side band transmission.

The nature of the time lag deviceswillf be understood from considering Fig, 3. The telegraphone W is driven forward at a constant speed in any suitable manner beneath the recording and listening coils R L etc. The components representing one of the sub-bands, for example, the components transmitted through the filter F are applied torthe recording coil R so that a magnetic record ofthese components is made on the wire W. As the wire travels for-- ward, the recorded variations arebrought underneath the listening coil L and currents are set up in this coil corresponding to those in the coil R which produced the record. These components induced in the c011 L pass into the outgoing circuit and It will be seen, therefore, that the currents impressedon the coil R are, after a certain t1me lag, transmitted into the outgoing circuit by the coil L The time lag introduced 'into any sub-band is determined by the setting of the coils R and L along the line of travel of the wire W, on the assumption'that the wire maintains a constantaspeed. The record produced in the wire by the coil R is eflaced'by the erasing coil E I The same wire W. may serve for introducing time lags into a plurality of the For example, the recording coil R maybe connected to the. listening coil L nay be connected to 'ferent spacing'between the coils R and L from that between the coils R and L different time lags may readily be introduced into the different sub-bands. V f

It is not necessary that a'time lag devlce be employed for each of the sub-bands andit may be desirable to transmit one or certain of the sub-bands without introduc-' ing any time lag.- This may readily be done 3 i by simply omitting the corresponding time the filter F and By providing a diflag devices from in Fig. 1.

It will be understood that the actual delay introduced in any sub-band is not to be so great as to prevent carrying on a conversation at the ordinary rate. A very slight amount of delay in the "arious subbands is sufiicient to destroy. the intelligr bility. of the waves as a whole. The subband. from filter F for example, may be transmitted with no time lag, the sub-band from filter F 2 may have a time lag of 0.01.

of a second, for example, and the sub-band from the filter F may have a time lag of 0.03 of a second, for example. To anyone picking up the transmitted waveon a simple-detector, the receivedcomponents would appearo'n'ly as noise currents incapable of being understood. If the observer should attempt to receive the speech by filtering out a single one of the sub-bands, he would be defeated since the frequency components comprised in any sub-band are of too narrow range to reproduce intelligible speech.

The wave. transmitted from the station 'of Fig. 1 may be received and madeto reproduce intelligible speech by the circuit shown in Fig. 2. The'wave is received on the antennaRA and detected at D where any suitable detector, such as a vacuum tube detector, may be employed. 'A source 4' of continuous Waves of the carrier frequency is connected to the detector. The detected currents are impressed upon the circuit'5, to which are connected the filtersF F and F These filters may be duplicates of the filters F F and F respectively, employed at the transmitting station to analyze the the paths of the sub-bands speech. F orexample, with the time lags as sumed to have been introduced into the subbands selected by the filters F \and F of -F'ig. 1, the speech'nray be restored at the receiver by introducing a time lag in the sub-band from the filter F of .03 of a second,"a time lag' in the sub band transmit ted by the'filter F of .01.. of a second and no tirne lag'. in the other sub-band. The total delay introduce byg the transmitting and receiving timel ,9: 'devices in the transmitted mess age'is then .03 of a second. The restored sub-bands are impressed together onthe cir- Inpr'actice. amplification .will need 'to' be introduced at various points of the system,

' within the frequency limits of one ofthe but for the sake of simplicity, no attempt has been made to indicate'this in the drawmg.

It will be seen to be a feature of the invention that the provisions for obtaining secrecy are made entirely within the limits of the speechband and,therefore, they lie side bands of the modulated wave, so'that secrecy of communication is obtained when only a single side-band is transmitted between the two stations. By suppressing one of theside bands and the unmodnlated carrier. component, applicant-is enabled to save about of the amount of energy transmitted, since it has been found that the energy represented by a side band of a speech modulated wave represents only about of the total energy of the entire modulated wave. This feature makes the invention particularlysuitedfor long distance radio transmission, such as transatlantic communication. However, the invention may be applied equally well to wire transmission, either to'ordinary telephone transmission or to carrier telephone transmission, and the invention is therefore not to be construed as limited to any particular manner or type of transmission.

\Vha-t is claimed is 1. The method of secret transmission of waves, the frequency components of which comprise a considerable frequency range, which comprises dividing said waves into sub-bands of frequency components, producing relative time shifts in the sub-bands, and then transmitting all of the sub-bands over the same path. 3 2. The method of secret transmission of speech, comprising dividing the speech waves into sub-bands of frequency components, producing different relative time shifts in the different sub-bands, and transmitting the resultant sub-bands over the same path.

"' order of the components within the fre-' 3. The method of secret transmission of speech comprising. separating the frequency components of normal speech into sub-bands of frequency components, each sub-band by itself being incapable of reproducing intelligible speech, producing different. relative time shifts in the separate sub-bands, and transmitting the resultant sub-bands over the same path.

4. The method of secret transmission of speech comprising separating the frequency components of speech into sub-bands of frequency components, producing different relative time shifts in the separate sub-bands while maintaining the original frequency quen cy limits of each sub-band. and transmitting the resultant sub'bands, over the same path. p

5. The method of secret transmission of speech comprising dividing the speech waves into sub-bands of frequency components, producing different relativctime shifts in the different sub-bands, producing a carrier wave, modulatingsaid carrier wave by said relatively shifted sub-bands and transmitting the resulting modulated wave.

6. The method of secret transmission of speech comprising dividing the frequencycomponents of normal speed into sub-bands of frequency components, each sub-band by itself being incapable of producing intelligible speech, producing different relative time shifts in the separate'sub-bands, modulating a carrier wave in' accordance with the relatively shifted sub-bands, suppressing transmission of one of the two side bands. of the resulting modulated wave-and the unmodulated carrier component and transmit ting to a distance the remaining side band.

7. The method of receiving the sub-bands transmitted according to the method of claim 1 comprising separating,the'respective subrbands, producing relative time shifts in the separate sub-bands to restore their normal relations-and impressing the restored. sub-bands upon a common receiver.

8. The method of receiving s eech from the sub-bands transmitted accor ing to the method defined in claim 2 comprising separating the sub-bands, producing different. relative time shifts in the separate subbands to restore their normal relations, and impressing the restored sub-bands upon a common receiver.

9. The method of receiving speech from the modulated Wave transmitted" by the method defined in claim 5, comprising demodulating the wave toreproduce the subbands of speech frequency range, separating the sub-bands, producing different relative combining said side band with a continuous H wave of the carrier frequencyin a demodulating circuit to reproduce sub-bands of speech frequency range, producing difi'erent relative time shifts in the separate sub-bands to restore their normal relations, and impressing the restored sub-bands upon a re-.. ceiver.

11. In a telephone system, means to produce speech currents, a plurality of filters,

each arranged for selectively transmitting a different sub-band of the frequency components ofthespeech currents, means for transmitting the sub-bands over. a'comn on path, and means to alter the time of transmission of some of the sub-bands relative to the others.

12. In .a telephone system, a source of 5 carrier waves, means to modulatethe waves from said source in accordance with the subbands resulting from the transmitting arrangement of claim 11, means to suppress from transmission, one of the two side bands of the resulting modulated waves and the 10 j unmodulated carrier wave component and means to transmit the remaining'side band to a distance.

In witness whereof, I hereunto subscribe my name this'26th day of June A. D., 1923.

ROBERT C. MATHES. 

